1. Field of the Invention
The present invention relates to detecting and quantitatively analyzing the spectral content of a light wave. It concerns more particularly a stationary Fourier transform spectrometer including a photodetector with semiconductor elements responsive particularly to incident radiation and a two-wave stationary interferometer comprising a semi-reflecting diopter.
2. Description of the Prior Art
A Michelson interferometer comprising a wave splitter and two mirrors, for example, imposes an optical path difference between two majority waves by positioning one of the mirrors relative to the other in translation or rotation. The intensity of the wave resulting from the phase difference of the two majority waves reflected by the mirrors varies as a function of the displacement of the mirror and forms an interferogram on the detector. The interferogram represents the variation of the luminous intensity received by the detector as a function of the phase difference which enables deduction of the spectral intensity as a function of frequency after sampling and using a Fourier transform.
The photodetector is useful for detecting and quantitatively analyzing the spectral content of a light wave. The analysis contributes to the definition of the emission and reflection optical properties of a source of radiation, such as an observed scene, and of the propagation medium between that source and the photodetector. Carrying out the measurement at different times identifies the physical phenomena involved in the emission and propagation of the radiation and how they evolve over time. If the photodetector is coupled to an imaging system (imager), each point of the observed scene and the spectral signature thereof are associated and therefore perform simultaneous imaging and spectrometry.
Prior art stationary Fourier transform spectrometers are described in the paper “Performance limits of stationary Fourier spectrometers”, M. -L. Junttila et al., Journal of Optical Society of America, Vol. 8, No 9, September 1991, p. 1457-1462, and have proven advantages in terms of resolution, spectral bandwidth, radiometric sensitivity and acquisition rate. A Michelson interferometer with a semi-reflecting beam splitter and two mirrors, or a modified Mach-Zehnder interferometer with a semi-reflecting beam splitter and three mirrors, or a triangular interferometer with a semi-reflecting beam splitter and two facing mirrors splits an incident light wave into two reflected waves having a phase difference and optical path differences that generate an interferogram on the observation plane consisting of the photosensitive face of the detector. The interferogram reproduced in the observation plane is sampled by a linear or matrix set of individual photosensitive elements.
The analysis spectral range of spectrometers of this type is delimited by wavelengths λmin and λmax. The lower limit λmin is linked to the sampling step dδ of the interferogram by the following formula:λmin=2dδ
In practice, the upper limit is defined by the sensitivity of the detector, which is blind beyond the value λmax.
The spectral resolution dλ of the spectrometer for the analysis of a light wave of wavelength λ is related to the maximum optical path difference δmax created by the interferometer between the two waves by the following formula:dλ=λ2/(2δmax)
Such spectrometers offer high performance in theory, but are subject to practical implementation limitations, related in particular to the opto-mechanical adjustment of the interferometer. The interferometer proves complicated to adjust, in particular in radiation bands where the monitoring means are not visual. In the case of infrared spectrometry of scenes with a low luminous flux, it is necessary to cool the whole of the spectrometer to reduce heat contributions related to the spectrometer. These practical constraints intrinsic to the spectrometer, added to the external constraints linked to experimental, on-board or field conditions, lead to the production of spectrometer prototypes that are costly, fragile and bulky.
On the other hand, the linear or matrix detectors used in this type of spectrometer have benefited over the last decade from considerable technological advances that have culminated in the mass production of photosensitive strips and matrices comprising a large number of individual detectors of high performance.
To reduce the drawbacks of previous spectrometers caused by their interferometers, Omar Manzardo et al., according to their paper “Miniaturized time-scanning Fourier transform spectrometer based on silicon technology”, Optics Letters, Vol. 24, No 23, December 1999, p. 1705-1707, have sought to miniaturize an interferometer with a splitter and two mirrors to yield a microsystem having dimensions compatible with integrated circuit formats. One of the mirrors is moved linearly by an electrostatic comb actuator. This technology necessitates additional technology steps to design and produce the microsystem, which must then be coupled to a matrix of photosensitive elements of a detector.